1. Field of the Invention
The present invention relates to a device for implementing a method of granulating slag fines, in particular steelworks fines and especially with a view to recycling them.
2. Description of Prior Art
One of the final steps in steel production is its processing in a ladle in liquid form so as to purify it, in particular from the sulphurous or oxidised phases that are concentrated in the slag. For this purpose, lime, dolomite or fluidising agents (such as fluorspar—CaF2, alumina, calcium aluminates, etc) are added to the ladle. This processing step in a ladle is referred to as “secondary metallurgy”.
Waste results from this secondary metallurgy, referred to as class 2 waste, which does not constitute a major nuisance for the environment; this waste comprises for example CaO, MgO, SiO2, Al2O3, etc. Nevertheless this waste requires to be dumped or recycled. At the present time dumping is becoming less and less possible given the more and more strict regulatory constraints with regard to the environment. It is particularly the fines, particles with a size less than 10 mm, in particular with a size less than 5 mm, whose solution poses a problem.
During recycling in an electric furnace, a large part of the slag fines (<5 to 10 mm) is sucked into the flue gas discharge system and mixed with the furnace dust. The slag fines then take on heavy metals and other class 1 elements. This dust with a high heavy metal content must then be sent for dumping as class 1, which represents an additional cost of approximately 50% compared with class 2 dumping. This recycling option is all the more prohibitive as the difference in costs between class 1 and class 2 is tending to increase.
In the case of recycling by means of agglomeration belts well known to person skilled in the art, the presence of this dust may serve as seeds for the appearance of dioxins downstream, which often makes this method of eliminating fines impossible.
Moreover, the discharge of dust during the loading of furnaces, due to the ascending thermal current, constitutes a source of nuisance for the environment and a real danger to the operators.
An alternative solution to dumping these slag fines, respectful of the environment and the safety of the operators, consists of agglomerating this dust so as to make it possible to recycle it in the steelmaking process and to prevent its being given off at the time of charging of the furnace since the agglomerates formed are heavier than the dust. The most simple method and the least expensive is granulation, for example in a rotary dish or a drum.
Adding to the dust an aqueous suspension based on calco-magnesian products, with any optional additives, promotes the cohesion of the granules, which then have sufficient mechanical strength to prevent any subsequent emission of dust.
A steelworks with a capacity of 1,000,000 tonnes/year of steel may generate approximately 6,000 tonnes/year of secondary metallurgy slag dust, which represents an hourly production of less than one tonne of fines. Here fines or slag dust means the granulometric fraction of this slag with a size of less than 10 mm.
In order to process this small output of dust, it is possible to have recourse to a granulation unit with a size adapted to this small output, but this would consequently be not very productive and also expensive in terms of labour.
In order to reduce this labour cost, it would also be possible to provide a large installation, but it would be used sporadically. This solution is also not acceptable since the installation would often function transiently, starting up and stopping, the stoppage periods also being prejudicial to the correct functioning of the equipment. The investment cost would also be disproportionate compared with the requirements.
Since neither a large installation functioning sporadically nor a small one functioning in an unproductive and expensive fashion can be envisaged, the main problem to be resolved for the device according to the invention is therefore to procure an installation where the labour costs are low, like those of a large installation, whilst operating relatively continuously in order to reduce the operating downtime, and which is adapted to the small outputs of fines to be recycled in the steelmaking industry.